The U.S. Department of Health and Human Services' National Toxicology Program (NTP)
performed short- and long-term studies in mice that found a correlation between chromium-6 in drinking water and multiple cancers. The results and conclusions from these studies were peer-reviewed by external experts and cited in multiple scientific journals.
In addition,
scientists and environmental groups have submitted a comment letter 
summarizing how industry lobbyists and the California EPA have overestimated the costs and underestimated the benefits of regulating chromium-6 in drinking water. This letter also cites multiple peer-reviewed studies.
